In the manufacture of multiple-part printed products containing information that varies from one product to the next, such as express shipping waybills and direct mail solicitations, it is necessary to ensure that all of the parts of each product match before the products are put into use. An express shipping waybill, for example, typically consists of a multiple-ply form with a tracking number printed in human readable or bar code form, or both, on each ply. The tracking number changes from form to form, but is the same on all plies of a given form. If the tracking numbers on all plies of a given form do not match due to an error in assembling the form, it may be difficult or impossible to track a lost shipment. Similarly, a direct mail solicitation typically consists of several parts, including a letter to the recipient, an order or application form that is intended to be returned by the recipient, and an envelope in which these items are enclosed. All of these parts will ordinarily be printed with the recipient's name and address, and possibly with other information that is unique to the recipient, and hence it is important to be sure that all parts intended for the same recipient are properly matched before the direct mail solicitation is sent. If this does not occur, the solicitation may have little or no value to the sender.
Multiple-ply shipping waybills and other types of multiple-ply forms are usually manufactured in a two-step process. In the first step, continuous webs or strips, one for each ply of the resulting form, are printed with the fixed information which does not vary from one form to the next. This information will typically consist of instructions for using the form, an arrangement of blocks or spaces in which information concerning the shipment is inserted, and the shipping company's name and logo. In the second step, rolls of the printed webs are mounted on a pin band collator for assembly into forms. In the collator, the individual webs are overlaid and adhesively bonded to each other to form a continuous strip of connected, multiple-ply forms which are then cut or perforated to produce individual forms. The required variable information (i.e., tracking numbers) can be printed on the individual plies of each form in two different ways. In one way, a mechanical numbering head or other type of variable printer is mounted on the printing press that is used for printing the fixed information on the continuous webs, so that the fixed and variable information is printed at essentially the same time. In another technique, the variable information is applied to the continuous webs as they are being unwound from the pre-printed rolls and fed to the collator, so that printing of the variable information and collating of the webs takes place in one continuous operation. In both methods, proper assembly of the printed forms requires that accurate registration be maintained among the various webs that are fed to the collator.
In the past, the function of maintaining proper web registration on a collator was carried out manually by a human operator. In the case of webs pre-printed with the desired variable information, the operator was required to perform initial registration of the webs when the pre-printed rolls were mounted on the collator, and to visually monitor the operation of the collator to ensure that registration was not lost due to a feed error or other malfunction. A loss of registration generally required that the collator be stopped and that one or more of the webs be repositioned to restore proper registration. For webs that were printed with the desired variable information as they were being fed to the collator, restoration of proper registration also required that the printing devices themselves be stopped and resynchronized.
For a number of reasons, reliance on manual methods for monitoring and restoring registration among multiple webs or plies in a collating machine is undesirable. For example, a human operator will not necessarily be able to detect and correct a registration error as soon as it occurs, particularly at high web speeds. Consequently, the error may persist for an undesirably long period of time and may result in a large volume of wasted product. Moreover, once a registration error is discovered, manual restoration of proper registration generally requires that the operation of the collator be stopped. This reduces product throughput and, if the collator is so equipped, can also cause clogging of the adhesive spray nozzles that are used to secure the plies of a form to each other. An additional problem with manual methods arises when the variable information that has been printed on the plies of the form consists in whole or in part of bar codes or other machine-readable indicia, since it can be difficult for a human operator to visually match these codes without a high degree of training or experience.
A number of systems have been proposed in which video cameras are used to automatically check for proper matching between different parts of a printed product, such as different personalized components of a mass mail item. These systems allow matching errors to be detected very quickly. However, video cameras must be triggered at precisely the right time in order to capture the desired image within the frame or field of view of the camera, and this imposes strict timing and synchronization requirements if the system is to operate properly. In known systems employing intermittently moving transport mechanisms, the fixed cycle time of the transport mechanism provides a reference that enables the video cameras to be triggered at the proper moment. However, the collators used to assemble multiple-ply forms generally operate continuously rather than intermittently, and hence time-based synchronization techniques are not practical in this situation. In one known type of video camera verification system designed for use with a continuously moving transport system, an encoder is used to synchronize the video cameras with the movement of the transport system. In this system, the encoder must turn one revolution for each form length or multiple thereof. Thus, if the form length changes, the gearing on the encoder must also change. Similarly, if a web break requires that the web be removed from the transport system and then replaced at a slightly different registration position, the video camera trigger points must be reprogrammed to correspond with the new form position. A further problem with this system is that, lacking any direct knowledge of the web position except by reference to the encoder output, the video cameras may be triggered at the wrong times if the web does not move in exact synchronism with the transport system. This would not ordinarily be a problem in a pin band collator, since the engagement of the pin bands with the webs provides the necessary synchronous motion, but it may occur in printing presses and other systems where there is no direct engagement between the transport mechanism and the web or sheet being processed. In the latter situation, paper stretching and web slippage may result in accumulating registration errors, and this may cause improper triggering of the video cameras used for verification or matching.
Video cameras are a useful type of scanning device in that they are capable of sensing virtually any type of visible indicia, including alphanumeric characters. However, in cases where the information to be matched consists of bar codes or other machine-readable indicia, the use of other, more specialized types of scanners or detectors is often preferable. For example, although a video camera is capable of detecting and decoding a printed bar code if appropriate software is provided, a dedicated laser bar code scanner can do so much more quickly and reliably, at lower cost, and with reduced set-up time. A bar code scanner is also capable of scanning a printed bar code at higher web speeds than a video camera, because of the time required to process the image data received from a video camera. Although it is possible to design a verification or matching system for use with bar code scanners rather than video cameras, this does not provide the user with the option to select either type of imaging device depending on the type of codes or indicia to be scanned at any given time. Bar code scanners and video cameras have different types of triggering and timing requirements, and a simple substitution of one type of device for the other is generally not possible.
Other drawbacks also exist with known types of automatic verification or matching systems. For example, known systems are generally designed to stop the motion of the product conveyor or transport mechanism when a mismatch is detected, in order to allow the error to be corrected manually. Although manual intervention may be desirable in some circumstances, there are other situations (particularly where a continuously operating collator with adhesive-applying stations is being used) in which it would be preferable to correct the registration error without stopping the conveyor or transport mechanism. Existing matching systems are also disadvantageous in that the source of a registration error is not always immediately apparent when the conveyor stops. This requires the operator to visually inspect the various feed stations to determine which station is supplying the part of the multiple-part product that does not match the others, so that the appropriate correction can be made. Ideally, it would be desirable to designate the source or cause of the registration error automatically, in order to reduce down time and operator workload. Another disadvantage of known matching systems is that it is not always possible to separate mismatched products from unaffected products after an error has occurred, and hence it is sometimes necessary to discard both in order to be sure that no defective products are inadvertently sold or put into use.